112 lines
3.6 KiB
C++
112 lines
3.6 KiB
C++
/*
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* Copyright (c) Facebook, Inc. and its affiliates.
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*
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* Licensed under the Apache License, Version 2.0 (the "License");
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* you may not use this file except in compliance with the License.
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* You may obtain a copy of the License at
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*
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* http://www.apache.org/licenses/LICENSE-2.0
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*
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* Unless required by applicable law or agreed to in writing, software
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* distributed under the License is distributed on an "AS IS" BASIS,
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* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
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* See the License for the specific language governing permissions and
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* limitations under the License.
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*/
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#pragma once
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#include <memory>
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namespace folly {
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/*
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* folly::enable_shared_from_this
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*
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* To be removed once C++17 becomes a minimum requirement for folly.
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*/
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#if __cplusplus >= 201700L || __cpp_lib_enable_shared_from_this >= 201603L
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// Guaranteed to have std::enable_shared_from_this::weak_from_this(). Prefer
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// type alias over our own class.
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/* using override */ using std::enable_shared_from_this;
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#else
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/**
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* Extends std::enabled_shared_from_this. Offers weak_from_this() to pre-C++17
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* code. Use as drop-in replacement for std::enable_shared_from_this.
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*
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* C++14 has no direct means of creating a std::weak_ptr, one must always
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* create a (temporary) std::shared_ptr first. C++17 adds weak_from_this() to
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* std::enable_shared_from_this to avoid that overhead. Alas code that must
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* compile under different language versions cannot call
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* std::enable_shared_from_this::weak_from_this() directly. Hence this class.
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*
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* @example
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* class MyClass : public folly::enable_shared_from_this<MyClass> {};
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*
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* int main() {
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* std::shared_ptr<MyClass> sp = std::make_shared<MyClass>();
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* std::weak_ptr<MyClass> wp = sp->weak_from_this();
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* }
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*/
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template <typename T>
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class enable_shared_from_this : public std::enable_shared_from_this<T> {
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public:
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constexpr enable_shared_from_this() noexcept = default;
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std::weak_ptr<T> weak_from_this() noexcept {
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return weak_from_this_<T>(this);
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}
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std::weak_ptr<T const> weak_from_this() const noexcept {
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return weak_from_this_<T>(this);
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}
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private:
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// Uses SFINAE to detect and call
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// std::enable_shared_from_this<T>::weak_from_this() if available. Falls
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// back to std::enable_shared_from_this<T>::shared_from_this() otherwise.
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template <typename U>
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auto weak_from_this_(std::enable_shared_from_this<U>* base_ptr) noexcept
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-> decltype(base_ptr->weak_from_this()) {
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return base_ptr->weak_from_this();
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}
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template <typename U>
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auto weak_from_this_(std::enable_shared_from_this<U> const* base_ptr) const
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noexcept -> decltype(base_ptr->weak_from_this()) {
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return base_ptr->weak_from_this();
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}
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template <typename U>
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std::weak_ptr<U> weak_from_this_(...) noexcept {
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try {
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return this->shared_from_this();
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} catch (std::bad_weak_ptr const&) {
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// C++17 requires that weak_from_this() on an object not owned by a
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// shared_ptr returns an empty weak_ptr. Sadly, in C++14,
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// shared_from_this() on such an object is undefined behavior, and there
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// is nothing we can do to detect and handle the situation in a portable
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// manner. But in case a compiler is nice enough to implement C++17
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// semantics of shared_from_this() and throws a bad_weak_ptr, we catch it
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// and return an empty weak_ptr.
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return std::weak_ptr<U>{};
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}
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}
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template <typename U>
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std::weak_ptr<U const> weak_from_this_(...) const noexcept {
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try {
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return this->shared_from_this();
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} catch (std::bad_weak_ptr const&) {
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return std::weak_ptr<U const>{};
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}
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}
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};
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#endif
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} // namespace folly
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